1. Field of the Invention
The present invention relates to a cable modem tuner. More specifically, the present invention relates to a cable modem tuner suitable for outputting an intermediate frequency signal to a QAM (Quadrature Amplitude Modulation) demodulating circuit.
2. Description of the Background Art
In a cable television system (hereinafter referred to as CATV), introduction of HFC (Hybrid Fiber/Coax) has been in progress, in which a coaxial cable is kept as a subscriber's drop wire and the main network is implemented by optical fibers. This system attempts to provide broad-band data communication service of several Mbits/sec at home. Utilizing this system, it is possible to realize high speed data line having the transmission rate of 30 Mbits/sec with the bandwidth of 6 MHz using 64 QAM (Quadrature Amplitude Modulation). The cable modem is used in this system, and realizes high speed data communication of 4 Mbits/sec to 27 Mbits/sec, utilizing an unused channel of CATV. The cable modem tuner is used for a cable modem in such a CATV system, and after the received CATV signal is subjected to frequency conversion, it serves to take out the signal as an intermediate frequency signal.
FIG. 8 is a block diagram representing a configuration of a conventional cable modem tuner 1. Referring to FIG. 8, as for the CATV signals, an up signal transmitted to a station has the frequency of 5 MHz to 42 MHz, and a down signal transmitted from the station to the cable modem tuner has the frequency of 54 MHz to 860 MHz, and transmitted to a cable network through an input terminal 2 of the tuner. The up signal transmitted from the cable modem is received by a data receiver of the CATV station (system operator), and enters a computer of a center.
The cable modem tuner 1 includes a CATV signal input terminal 2 receiving the CATV signal as an input, a data terminal 3 receiving a data signal from a QPSK transmitter as an input, and an upstream circuit 4 provided between data terminal 3 and CATV signal input terminal 2.
In the cable modem, a data signal subjected to quadrature phase shift keying (QPSK) from a QPSK transmitter, for example, is input to data terminal 3 as the up signal. The data signal is transmitted through upstream circuit 4 to the CATV station.
The down signal input through input terminal 2 is divided into a UHF band receiving the frequency of 470 to 860 MHz, a VHF-High band receiving the frequency of 170 to 470 MHz and a VHF-Low band receiving the frequency of 54 to 170 MHz, and processed by receiving circuits provided for respective bands. Band ranges are not limited to those specified above.
The cable modem tuner 1 further includes a high pass filter (HPF) 5 having an attenuation range of 5 to 46 MHz and a pass band of not lower than 54 MHz, and input switching circuits 6 and 7 for allocating the signals passed through the high pass filter 5 to circuits corresponding to respective bands. The down signal is passed through high pass filter 5, the band is switched by the input switching circuits 6 and 7, and supplied to the circuitry corresponding to any of the aforementioned bands.
Cable modem tuner 1 further includes high frequency amplification input tuning circuits 8, 9 and 10 provided corresponding to respective bands; high frequency amplification circuits 11 and 12 provided corresponding to the UHF band and VHF band, respectively; high frequency amplification output tuning circuits 15, 16 and 17 provided corresponding to respective bands; a mixer circuit 18 and a local oscillation circuit 19 provided corresponding to the UHF band; a mixer circuit 20 and a local oscillation circuit 21 provided corresponding to the VHF band; and an intermediate frequency amplifying circuit 22 for amplifying, in the intermediate frequency band, outputs from mixer circuits 18 and 20.
The high frequency amplification input tuning circuits, the high frequency amplification circuits, the high frequency amplification output tuning circuits, the mixer circuits and the local oscillation circuits provided corresponding to respective bands are adapted such that dependent on the received channel, circuits corresponding to the received band are made operative, while the circuits corresponding to other bands are made inoperative. For example, when a UHF channel is received, the high frequency amplification input tuning circuit 8, the high frequency amplification circuit 11, high frequency amplification output tuning circuit 15, mixer circuit 18 and local oscillation circuit 19 for the UHF band are set to the operative state, while high frequency amplification input tuning circuits 9 and 10, high frequency amplification circuit 12, high frequency amplification output tuning circuits 16 and 17, mixer circuit 20 and local oscillation circuit 21 for the VHF-High and VHF-Low bands are set to inoperative state, and stop their operation.
The CATV signal input to the input terminal 2 is passed through high pass filter 5, and enters input switching circuits 6 and 7, where band switching takes place. The output therefrom is fed to high frequency amplification input tuning circuit 8, 9 or 10, where channel selection takes place. After channel selection, the signal is input to AGC terminal 24, amplified to a prescribed level by high frequency amplification circuit 11 or 12 based on the AGC voltage applied to resistance 13 or 14, supplied to high frequency output tuning circuit 15, 16 or 17, where the received signal is extracted.
Thereafter, the selected received signal is subjected to frequency conversion by mixer circuit 18, 20 and local oscillation circuit 19, 21 to an intermediate frequency (IF), and amplified by intermediate frequency amplification circuit 22. The intermediate frequency signal amplified by intermediate frequency amplifying circuit 22 is output from output terminal 23.
In this manner, in the conventional cable modem tuner 1, a received CATV signal is selected in accordance with the reception channel, and the signal after channel selection is subjected to frequency conversion and output as an IF signal from output terminal 23.
Handling of a QAM signal, which is a digital signal, and transmitting the IF signal output from output terminal 23 to a QAM demodulating circuit, not shown, for QAM demodulation by using such a cable modem tuner 1 has the following various problems.
First, it is necessary to input a high level IF signal of 50 dB or higher to an IC formed as a QAM demodulating circuit. In a recent QAM demodulating IC, performance of an ADC (Analog to Digital Converter) therein is improved, and it has become a general practice that the IC receives as an input a high level intermediate frequency signal. Further, an input circuit of the QAM demodulating IC contains an IF amplifying circuit and a fixed attenuation circuit. Therefore, performance has been improved at the level of ½ (0.5Vp-p), as compared with the conventional example. In the conventional single conversion tuner, however, the gain is around 30 dB, which is an insufficient level.
Further, generally, a balanced type input circuit comes to be used in the QAM demodulating ICs. This means that the tuner must provide balanced IF outputs. Conventionally, however, it provides unbalanced outputs.
Further, generally, an input signal level of about 0.5Vp-p is necessary for the QAM demodulating IC. The IF output of a conventional tuner, however, does not have such a level.
Further, the input level of the QAM demodulating IC must be constant for reception signals. In the conventional tuner, what is provided is RF-AGC only, and variation of the reception signal level at the tuner input end cannot be controlled satisfactorily.
Further, as the input level of the QAM demodulating IC is high, a high gain IF amplifying circuit is necessary. This leads to a problem that CPU clock noise and bus noise tend to enter, as QAM demodulating IC, CPU, IF amplifying circuit and the down converter circuit exist on one same substrate.